Method and system for ion beam delayering of a sample and control thereof

ABSTRACT

There is provided a method, system and computer program product to delayer a layer of a sample, the layer comprising one or more materials, in an ion beam mill by adjusting one or more operating parameters of the ion beam mill and selectively removing each of the one or more materials at their respective predetermined rates. There is also provided a method and system for obtaining rate of removal of a material from a sample in an ion beam mill.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of co-pending U.S. ProvisionalPatent Application No. 61/558,418, filed 10 Nov. 2011, which is herebyincorporated herein.

FIELD OF THE INVENTION

The present disclosure relates generally to delayering samples using ionbeam mills, and in particular, using broad ion beam mills to delayer alayer of a sample, by selectively removing one or more materials of saidlayer, and control thereof.

BACKGROUND OF THE INVENTION

Removing a layer in a sample such as a semiconductor die involvesremoving very small amounts and very thin layers of an integratedcircuit, which contains metals and dielectrics, to reveal the underlyingcircuitry in a precise and controlled manner. Typical methods includewet chemical etching, dry (plasma) etching, and mechanical polishing orphysical abrasion.

Mechanical polishing is performed by manually polishing the sample usingpolishing pads and abrasive slurries to erode the surface of the sampleto the extent needed. The problem faced during this process is theuneven erosion of the periphery and surface, wherein for example, copperis removed slower than SiO₂. This leads to the non-uniform removal of agiven surface, due to various levels of stress exerted in differentspots, or variations in feature density of the sample during labouringthe sample.

Wet (chemical) etching is performed by using chemicals and immersing asample into the chemical to cause a chemical reaction to remove materialfrom the sample surface. This is very difficult to control as the ratesat which the chemicals etch the various materials in the sample vary,and material interfaces can be severely affected, which once again leadsto the non-uniform removal of materials.

Dry (plasma) etching is performed by using combinations of non-reactivegasses and/or reactive gasses, ionized under vacuum in a strong electricfield. Reactive ions cause both chemical reactions on a sample andphysical bombardment, thereby removing material from the sample, whereasnon-reactive ions cause only physical bombardment of the sample andthereby eroding (knocking-off) the sample. The non-uniformities inmaterial density and etch species concentration adversely affect theetch rate and subsequent removal processes.

Ion beam milling is also used for material removal in samples by etchingor milling a sample. Ion beam mills may be used for various otherpurposes in the semiconductor industry, such as film deposition orsurface modification or activation. Using an ion beam source with bothreactive and non-reactive gases, the source gas is ionized and thepositive ions are extracted and accelerated toward the sample residingon a rotatable cooled stage in a vacuum chamber. The angle of the samplestage can be adjusted for the desired impact of the ions on the surfaceof the sample. There are various Ion Milling systems known in the art,such as Focussed Ion Beam Milling (FIB) systems and Broad Ion BeamMilling (BIB) systems.

Very narrow (small diameter) ion beams, typically with gallium ions, areused in FIB systems to remove material in precise locations in a sample(often on semiconductor integrated circuits) and also to deposit newmaterials on the ICs. This is used to edit the circuits, reroutingconnections to repair damage or introduce new functionality. FIB systemsare also used to cross section samples, build novel physical structures,and physically shape material (micromachining) on a very small scale. Atypical area shaped by the ion beam would be measured in microns, or atmost, tens of microns. The sample is kept stationary, while the ion beamis scanned back and forth. Beam to sample angle can be controlled bytilting the sample. The target areas capable of being practicallymodified by FIB are restricted to small, due to the relatively slowmilling rate of FIB systems. In addition, there are a number of otheraspects relating to a small scanned beam that make it quite difficult toaccurately modify large areas, including dwell time, overlap area,proximity between scans and features, that are all exacerbated as, forexample, the very narrow beam is passed over the entire surface of asample (such as an integrated circuit).

Medium diameter ion beams (millimeter sized) are typically used to‘clean up’ a sample, removing surface damage generated in previoussteps. One example is during transmission electron microscopy (TEM)sample preparation; a sample is polished using physical abrasives untilit is very thin, then a medium diameter ion beam (often using Argonions) is used to abrade the surface and gently mill away a thin layer(of nanometers thickness). The beam is kept stationary while the sampleis typically rotated or scanned back and forth, or both. Beam to sampleangle is usually adjustable by moving the ion gun. Milled area ismeasured in hundreds of microns, or in millimeters.

Finally, broad ion beam milling systems (centimeters in diameter) arealso used in the fabrication process of semiconductor devices. A layerof a sample is masked, when the sample is exposed to the beam, materialis removed over a large area where not protected by the mask. The gun isstationary but the sample can be rotated and tilted to different angles.Milled area is measured in centimeters. The material removed istypically homogenous in nature (a layer of a single material or singlecompound is milled until removed). BIB mills have been limited toremoving a layer of homogenous material as the removal rate ismaintained constant for a given homogenous layer until the next layer isreached. BIBmilling ion guns are associated with “grids” or “fields” infront of the ion gun that are capable of changing parameters of thebeam. Typical beam spreads in broad beam ion gun applications are in therange of 5 to 20 cm. Typically, broad ion beam applications inIntegrated Circuits (IC) include deposition and de-layering whenbuilding structures on an IC.

In deposition applications, broad ion beams are directed at a materialsource. The ion beam bombards the material source and causes the atomsof the material source to be ejected therefrom. A substrate is placed ina location where the ejected material source will hit and bond as alayer thereto in a more or less even fashion. The substrate can be movedlinearly (in x, y and z directions) and rotated (about x, y and zaxes—which would include a change in tilt angle of the substrate,relative to the main [?] direction of impact of the ejected materialsource). A mask can be used to create pre-defined structures on thesubstrate. Alternatively, material can be deposited on the maskbeforehand in a predefined pattern that, when removed, causes thedeposited material to remain on the substrate in a negative image of thepredefined pattern.

In material removal applications, broad ion beams are directed at asample in order to remove sample material in a non-selective manner.Generally, when a mask is pre-applied to the sample or a maskingmaterial is deposited on the sample beforehand in a predefined pattern.Known systems are directed to unselectively remove homogenous materiallayers of the sample without eroding the mask or the sample under themask to facilitate creation of structures on an IC. The angle of thesample may be adjusted to maximize the removal rates for a substantiallyhomogenous material layer. An endpoint detection system may also be usedto detect when the substantially homogenous material layer has beensubstantially removed and the material from a subsequent layer is beingremoved, at which point removal is stopped.

U.S. patent application Ser. No. 11/205,522, discloses a “Method ofReworking Structures Incorporating Low-K Dielectric Materials”. U.S.patent application Ser. No. 11/661,201 discloses “DirectedMult-Deflected Ion Beam Milling of a work Piece and Determining andControlling Extent Thereof”. Further, U.S. Pat. No. 5,926,688 discloses“Method of Removing Thin Film Layers of a Semiconductor Component”.However, none of the noted patent or patents overcomes the shortcomingsin the general area of delayering a sample.

Therefore there is a need for a method and system to overcome some ofthe shortcomings in the general area of delayering a sample.

This background information is provided to reveal information believedby the applicant to be of possible relevance to the present technology.No admission is necessarily intended, nor should be construed, that anyof the preceding information constitutes prior art against the presentinvention.

SUMMARY OF THE INVENTION

An object of the present technology is to provide a method and systemfor ion beam delayering of a sample and control thereof.

In accordance with an aspect of the present technology there is provideda method of delayering a layer of a sample using an ion beam mill,wherein the layer comprises one or more materials, the methodcomprising: placing the sample in the ion beam mill; operating the ionbeam mill; adjusting one or more operating characteristics of the ionbeam mill; and selectively removing each of the one or more materials atrespective predetermined rates.

In accordance with another aspect of the present technology there isprovided a method of delayering a sample, the method comprising:providing a broad ion beam mill; placing the sample inside a processingchamber of said ion beam mill; controlling said ion beam mill; andselectively removing one or more materials in a layer of said sample.

In accordance with another aspect of the present technology there isprovided a system for delayering a layer of a sample, wherein the layercomprises one or more materials, the system comprising: an ion beammill; and a control system in operative communication with said ion beammill to control one or more operating characteristics of the ion beammill to selectively remove each of the one or more materials atrespective predetermined rates.

In accordance with another aspect of the present technology there isprovided a system for delayering a sample, the system comprising: abroad ion beam mill; and a control system in operative communicationwith said broad ion beam mill, wherein said control system controls saidbroad ion beam mill to selectively remove one or more materials in alayer of said sample placed inside a processing chamber of said broadion beam mill.

In accordance with another aspect of the present technology there isprovided a computer program product for delayering a layer of a sample,wherein the layer comprises one or more materials, the computer programproduct comprising code which, when loaded into memory and executed onone or more processors, is adapted to control a system, said systemcomprising an ion beam mill, to selectively remove each of the one ormore materials at respective predetermined rates.

In accordance with another aspect of the present technology there isprovided a computer program product for delayering a sample, thecomputer program product comprising code which, when loaded into memoryand executed on one or more processors, is adapted to control a system,said system comprising a broad ion beam mill, to selectively remove oneor more materials in a layer of said sample placed inside a processingchamber of said broad ion beam mill.

In accordance with another aspect of the present technology, there isdisclosed method of reverse engineering a sample using an ion beam mill,wherein the sample comprises one or more materials, the methodcomprising the steps of placing the sample in the ion beam mill;operating the ion beam mill; adjusting one or more operationalcharacteristics of the ion beam mill to selectively remove each of theone or more materials at respective predetermined rate; removing a layerof constant thickness from a top surface of the sample; and acquiringsurface data from the top surface of the sample. The surface data maycomprise a picture, image or other data representation capable ofcharacterizing the features or other aspects of the top surface. Themethod may optionally be performed wherein the step of removing thelayer of constant thickness is achieved in a single step, wherein thepredetermined rate of removal for each material present in the layer ofconstant thickness is the same, or it is repeated multiple times,wherein each repeat the respective rate of removal for each material maybe different but the result of the repeated steps, each with the ionbeam mill at different operating characteristics, results in removing alayer of constant thickness. Optionally, the method may further comprisethe step of repeating the aforementioned steps until either apredetermined number of layers or predetermined total thickness of thesample, both as predetermined by the operator, have been removed fromthe sample. Optionally, the method may further comprise the step ofproducing hierarchical circuit schematics using the acquired surfacedata from each removed layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present technology will become apparentfrom the following detailed description, taken in combination with theappended drawings, in which:

FIG. 1, in aspects, is a representation of the various layers that maybe present in an IC (Prior Art).

FIG. 2, in aspects, is a representation of an ion gun, ion beam, samplestage and a sample (Prior Art).

FIG. 3, in aspects, is a graph representing material mill rate versus anion mill operating parameter for two materials A and B.

FIG. 4, in aspects, is a graph representing material mill rate versussample stage angle for Copper (Cu) and Silicon Dioxide (SiO₂).

FIG. 5, in aspects, is a graph representing material mill rate versussample stage angle for Copper (Cu), Silicon Dioxide (SiO₂) and SiliconNitride (Si₃N₄).

DETAILED DESCRIPTION OF THE INVENTION Definitions

Sample: A sample may refer to a composition of one or more materials. Asample may also refer to, but is not limited to: a semiconductor device,Integrated Circuit, a layer of metals and dielectrics of any thickness,one or more materials in an area of any size, optical devices,electronic devices, or any combinations thereof. A worker skilled in theart would readily understand the meaning of a sample for the purposes ofthe subject matter disclosed herein.

Delayering may entail, but is not limited to: removal of one or morelayers, partly or wholly, wherein the one or more layers or portionsthereof may comprise one or more materials; removal of one or morelayers, partly or wholly, comprising one or more materials, wherein theone or more layers may comprise small or large surface areas; removal ofone or more layers, partly or wholly, wherein the one or more layers maybe of any desired thickness; removal of one or more materials, partly orwholly, to any extent desired; removal of one or more substantiallyparallel layers, partly or wholly, wherein the one or more substantiallyparallel layers layers or portions thereof may comprise one or morematerials; removal of one or more substantially planar layers, partly orwholly, wherein the one or more substantially planar layers or portionsthereof may comprise one or more materials; removal of one or moresubstantially constant thickness parallel layers, partly or wholly,wherein the one or more substantially constant thickness parallel layersor portions thereof may comprise one or more materials; removal of oneor more varying thickness parallel layers, partly or wholly, wherein theone or more varying thickness parallel layers or portions thereof maycomprise one or more materials or any combinations thereof. For thepurposes of the subject matter disclosed herein, the terms delayeringand de-layering may be used interchangeably.

Homogenous, for the purposes of this disclosure, it is used to describematerials, structures, compositions or portions thereof, which comprisesolely one material.

Non-Homogenous, for the purposes of this disclosure, it is used todescribe materials, structures, compositions or portions thereof, whichcomprise more than one material.

As used herein, the term “about” refers to a +/−10% variation from thenominal value. It is to be understood that such a variation is alwaysincluded in a given value provided herein, whether or not it isspecifically referred to.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by a worker of ordinaryskill in the art to which this invention pertains.

In the instant application there is provided a method, system andcomputer program product to delayer a layer of a sample, the layercomprising one or more materials, in an ion beam mill by adjusting oneor more operating parameters of the ion beam mill and selectivelyremoving each of the one or more materials at their respectivepredetermined rates.

Delayering a Sample in an Ion Beam Mill:

Delayering may be done using various ion beam mills, such as FIB millsor BIB mills. In aspects of the subject matter disclosed herein,delayering of a sample is performed in a BIB mill. An ion beam mill maybe used to remove one or more layers of an IC, layer by layer, to exposethe underlying circuitry across the entire surface of the IC. Forexample, as seen in FIG. 1, an IC may have many layers. The bottom mostlayer may be a poly layer 101. Following the poly layer may be a numberof metal layers 102 to 105. A worker skilled in the art would readilyunderstand the layers within an IC.

According to some embodiments of the invention, a layer of a sample orportions thereof may be made up of one or more materials. Removal of alayer may comprise, partly or wholly, removing one or more materialsresident within. A layer may be of any thickness. One or more materialspresent in a sample may be any one or more materials from the chemicalperiodic table. Each layer may be made up of a mixture of materials suchas, but is not limited to, metals and dielectrics in varying shapes andstructures. There are a wide number of ion beam mills readily availablein the market as would be known to a worker skilled in the art.

For the purposes of this disclosure, ion beam mill, mill and ion millmay all be used interchangeably. Similarly, Broad Ion Beam mill, BIBmill and Broad Beam Ion mill may all be used interchangeably.

An ion beam mill may consist of one or more ion beam sources. Accordingto some embodiments of the invention, the ion mill consists of one ormore large diameter gridded ion beam source and a, variable angle,cooled sample stage that can be tilted and rotated. A sample stage maybe housed in a vacuum chamber. Various gas injection systems may deliverdifferent process gasses, while a plasma bridge neutralizer may be usedto neutralize the ion beam. Vacuum gauges, a load-lock, vacuum pumps,one or more control panels, and one or more processors may also beassociated with the ion mill. The ion mill may also comprise elements tointroduce various gasses and one or more elements to control the samplestage. Furthermore, one or more ion beam sources may be associated withapertures and electrostatic lenses. The ion mill may consist of one ormore actuators to actuate one or more elements of the ion mill. The ionmill may comprise one or more controllers or a control system comprisingone or more processors to control the one or more ion mill operatingcharacteristics or parameters. It is to be understood that the operationof an ion mill and the various fundamental components of an ion millwould be readily known to a worker skilled in the art. FIG. 2 showsillustrative example of an ion source 201 from which an ion beam 202 isproduced. The ion beam impinges on a sample 204, which is placed on asample stage 203.

According to some embodiments of the invention, the ion mill and itsvarious components may be collectively referred to as the ion mill. Theion mill may comprise an end point detection unit as one of itscomponents. The end point detection unit may be in operative connectionwith the ion mill. The end point detection unit may be capable ofdetecting or measuring one or more changes in the sample as one or morematerials are being removed. For example, the detection unit may detector measure the existence and/or quantity of different materials. The ionmill may comprise an imaging system as one of its components. Theimaging system may be in operative connection with the ion mill. Aworker skilled in the art would readily know various end point detectionunits and imaging systems. Furthermore, the ion mill together with acontrol system may form a system to delayer a sample.

Generally, delayering a sample in an ion beam mill involves an ion beamincident and impinging upon a surface of the sample whereby interactionof the ion beam with the surface results in removal of material from thesurface at a rate. For the purposes of the subject matter disclosedherein, the terms rate, material removal rate, removal rate, delayeringrate, milling rate, etch or etching rate, material rate, and rate ofremoval may all be used interchangeably.

The surface of the sample may be non-homogenous and thereforeconstitutes different compositions of materials. The surface of thesample may also be homogenous, which constitutes a single materialcomposition. The removal of material from the surface may be viaselectively removing one or more of the materials resident on thesurface. The removal of material from the surface may be vianon-selectively removing one or more materials without having anycontrol over which material is being removed from the surface. Upondelayering a surface of a sample, the underlying surface may be leftsubstantially uniform or even regardless of the delayered surface beinghomogenous or non-homogenous. Upon delayering a surface of a sample, theunderlying surface may also be left substantially non-uniform or uneven.

According to some embodiments of the invention, the ion beam mill isconfigured by adjusting one or more of its operating characteristics.The one or more ion beam mill operating characteristics may beassociated with a predetermined rate at which a material may be removed.Delayering a sample may be achieved by configuring the ion mill toremove one or more materials from the sample at their respectivepredetermined rates. The association of rates of removal to sets of ionmill operating characteristics may be obtained experimentally throughtrial and error or via simulation methods. The rates of removal andtheir associated sets of ion mill operating characteristics may belogged or stored for future manipulation of the ion mill in any storagemedium such as a database, memory device, computing storage device orany storage medium as would be known to a worker skilled in the art.

Delayering may be set to take place for a certain time; after which, thesample may be removed from the ion beam mill, analyzed, and furtherdelayering necessitated, until the desired level of delayering isachieved.

According to some embodiments of the invention, delayering may be set totake place until a certain thickness of the sample is delayered.Delayering may also involve removal of substantially parallel homogenousor non-homogenous layers. The surface area of the layer of the sample tobe delayered may be large. Furthermore, the surface area of the layer ofthe sample to be delayered may be in the range of 5 to 20 squarecentimetres.

According to some embodiments of the invention, a Secondary Ion MassSpectroscopy (SIMS) end point detection unit may be used to help withaccurately controlling milling rates. A worker skilled in the art wouldreadily understand SIMS and its usage as an end point detection element.Visual detection, chemical detection tools, other mass spectroscopytools or any end point detection system as would be readily known to aworker skilled in the art may also be used as an end point detectionunit/element to accurately control milling rates.

According to some embodiments of the invention delayering may beperformed for reverse engineering the circuitry inherent within adevice. An ion beam mill may be used to delayer a device layer by layerand exposing the circuitry or circuit connections on the surface of eachlayer. Upon delayering the device, pictures, images or otherrepresentation (e.g. circuit schematic model based on datarepresentative of detected surface features) may be taken of each layer,thereby, capturing the circuitry or circuit connections on the surfaceof each layer. By piecing together the pictures, images, or otherrepresentations of the different layers, using appropriate softwaretools, circuit connections between the various components that may beinherent within a device, both across and between layers, can beproduced. The process may be repeated for various devices within alarger device and a hierarchical schematic of the circuit connections ofthe various devices within the larger device may be developed.Proprietary software tools may also be used to produce hierarchicalcircuit schematics. Such circuit schematics may be useful in identifyingevidence of use of claim elements in the target device being delayered.According to some embodiments of the invention, delayering may beperformed for, but is not limited to, failure analysis (defectidentification), circuit edit, sample/device characteristicsmeasurement, verification of design, and counterfeit detection.

Selectivity:

The operation of an ion beam mill, as mentioned above, may depend on oneor more ion beam mill operating characteristics or parameters. Theoperation of an ion beam mill and the results of such operation may varybased on the adjustments made to one or more ion beam mill operatingcharacteristics.

According to some embodiments of the invention, the one or more ion milloperating characteristics or parameters may include, but are not limitedto, ion beam direction, ion beam size, sample cooling, chamber basepressure, chamber cross-over pressure, chamber process pressure, loadlock base pressure, load lock cross-over pressure, sample stage linearlocation, sample stage angle, sample stage rotation speed, sample stagetemperature, ion source accelerator voltage, ion source acceleratorcurrent, ion source beam voltage, ion source beam current, ion sourceextractor grid configuration, ion source extractor grid material, plasmabridge neutralizer (PBN) cathode voltage, plasma bridge neutralizeremission current, PBN gas flow, ion source gas flows and types,background gas flows and types, angle of sample-to-ion gun, any chambercondition, and sidewall angle or any combinations thereof or any ionmill characteristic or parameter that may be adjusted as would be knownto a worker skilled in the art. For the purposes of the subject matterdisclosed herein, ion mill operating characteristics, ion mill operatingparameters, characteristics, and parameters may be used interchangeably.Similarly, sample stage angle and sample angle may be usedinterchangeably. Beam angle, sample to beam angle or sample stage tobeam angle would all mean the angle at which the beam impinges on thesample.

According to some embodiments of the invention, the ion source radiofrequency power may be forward or reflected. The ion source extractorgrid configuration may include selecting the appropriate material, size,number and grid pattern of the extractor grids. The extractor gridconfiguration may be set to produce focussed, collimated or divergentbeam. According to some embodiments of the invention the ion source gasflows and types may be multiple. The ion source gas may be inert orreactive. Furthermore, the background gas flows and types may bemultiple. The background gas may be reactive. In addition, the angle ofsample-to-ion gun may be adjusted by moving either the sample stage orion gun or both. The angle of sample-to-ion gun may be adjusted toselect optimum etching/milling rates. According to some embodiments ofthe invention, the ion source extractor grid material may be graphite,molybdenum or any other material as would be readily understood by aworker skilled in the art.

According to some embodiments of the invention, adjusting the one ormore ion mill operating characteristics may provide different materialremoval rates. For example, for a given set of ion mill operatingcharacteristics the rate of removal for a particular material in aparticular sample or different samples may be the same. As anotherexample, a given material in a given sample may have different rates ofremoval for different sets of ion mill operating characteristics. As afurther example, a given material in different samples may havedifferent rates of removal for a given set of ion mill operatingcharacteristics. As yet another example, a given material in differentsamples may have different rates of removal for different sets of ionmill operating characteristics. Various permutations and combinations ofthe one or more ion mill operating characteristics may provide differentrates of removal for a given material in the same sample or in differentsamples. A set of ion mill operating characteristics or parameters mayentail a set of values corresponding to the one or more ion milloperating characteristics or parameters associated to the rate at whicha material may be removed.

According to some embodiments of the invention, the rate may bepredetermined. Predetermined rates may be obtained through trial anderror via experiments or via simulation methods. For a given material,the predetermined rate may be obtained by setting the ion beam millcharacteristics to a certain set of values and obtaining thecorresponding rate at which the material may be removed.

For example, a first predetermined rate for a particular material may beobtained by placing the material inside the ion mill and adjusting theoperating characteristics to a certain set of values and logging therate at which the material is removed. A second predetermined rate ofremoval for the same material may be obtained by adjusting the ion milloperating parameters to a second set of values. Furthermore, varioussets of ion mill operating characteristics, for a given material, may beset and the corresponding rates determined. In addition, variouscombinations of ion mill operating characteristics may be set andexperiments performed to obtain removal rates of one or more materialsin the same or different samples. Rates determined as such may then beclassified as predetermined rates. Predetermined rates may also beobtained via simulation methods. In aspects, predetermined rates ofmaterials may be obtained by introducing reactive species and mixingthem with matter ejected from the ion beam source. It is to beunderstood that any method of obtaining or measuring material removalrates would fall under the purview of this disclosure.

Predetermined rates for different materials may be associated with theirrespective sets of ion mill operating characteristics. The predeterminedrates for different materials and their associated respective sets ofion mill operating characteristics may be logged manually or stored inany medium and used for future manipulation of the ion mill. Accordingto some embodiments of the invention, each set of ion mill operatingcharacteristics may comprise one or more ion mill operatingcharacteristics. The values for the one or more ion mill operatingcharacteristics within each set may be the values that would have beenset during the trial and error or simulation process. In addition, asthese values may be set during the trial and error or simulationprocess, they may also be regarded as predetermined values. Furthermore,the medium used for storage of sets of ion mill operatingcharacteristics and associated respective predetermined rates is acomputer drive, electronic device, optical device, or any storage mediumthat would be readily known to a worker skilled in the art. The storagemedium may be part of the ion mill and the storage medium may be part ofa system comprising the ion mill and a control system.

According to some embodiments of the invention, selectively removing maybe regarded as adjusting the one or more ion mill operatingcharacteristics to remove a material, at a certain predetermined rate,relative to removal of another material at a different predeterminedrate. As well, selectively removing may be regarded as removing aspecific material. Non-selectively removing may be regarded as operatingthe ion mill with a certain set of operating parameters without havingcontrol over the rates of removal of the one or more materials. This maylead to non-uniform or uneven underlying surface.

According to some embodiments of the invention, the ion mill operatingcharacteristics may be set such that for a given time period, thepredetermined rate of removal of a first material is higher than that ofa second material. After which, the ion mill characteristics may beadjusted and set to another set of values, for a time period duringwhich the predetermined rate of removal of the second material is higherthan that of the first material. It may also be possible for thepredetermined rates of all materials to be removed to be substantiallysame.

For the purposes of this disclosure, period of time and time period maybe used interchangeably.

According to some embodiments of the invention, all but one of the oneor more ion mill operating parameters may be set. One of the one or moreion mill operating parameters may be variable and the rate of removal ofone or more materials for various values of the variable ion milloperating parameter may be obtained experimentally or via simulation.The results obtained and their respective ion mill operating parametersset to obtain them may be associated with each other and stored in astorage medium. A worker skilled in the art would readily understand thekind of storage medium needed. Based on the results obtained, a graph300 may be plotted as shown in FIG. 3. The plotted graph shows millrates or removal rates for two materials, A 310 and B 320, as a functionof an ion mill operating parameter. At 301, the rate of removal for B ishigher than the rate of removal for A. At 302, the rate of removal issubstantially equal for both A and B and at 303, the rate of removal ofA is higher than the rate of removal for B.

According to some embodiments of the invention, when encountered with asample, for delayering, comprising materials A and B, the delayering maybe set to take place with the one or more ion mill operating parametersset such that both material A and material B may be removedsimultaneously. Simultaneous removal of A and B may be obtained when theone or more ion mill operating parameters are set such that the rates ofremoval for both A and B are substantially equal as seen at theintersection point 302. For example, the one or more ion mill operatingparameters may be set such that material B is removed at a higher ratethan material A. Thus selective removal of materials from a sample maybe achieved. In such a delayering process, all but one of the ion milloperating parameters will stay constant for any rate of removal for thetwo materials A and B. The one ion mill operating parameter that isvariable may be set to the respective value as required. For example,for delayering at 302, the required parameter value may be the parametervalue corresponding to the intersection point. This would allow for bothmaterials, A and B, to be removed at the same rate.

According to some embodiments of the invention, one or more ion milloperating characteristics may be set and the rate of removal as afunction of sample stage angle, which may also be regarded as anoperating parameter of the ion mill, may be determined. Thedetermination of the rates of one or more materials may be experimental,or via simulation methods. Furthermore, any other ion mill operatingcharacteristic may replace the sample stage angle.

For example, the rates determined may be for two materials and the twomaterials may be Cu and SiO₂. The set of ion mill operatingcharacteristics that relate to the rates of removal for Cu and SiO₂ forvarious sample stage angles may be associated with one another,respectively, and stored in any storage medium. Using the predeterminedrates and the associated ion mill operating characteristics from thestorage medium, a graph 400 comprising the predetermined rates as afunction of sample stage angles may be plotted for both Cu 401 and SiO₂402. It is to be understood that delayering may be carried out withoutexplicitly plotting a graph. For example, a control system that maycomprise one or more processors may be able to control the ion mill todelayer a sample without explicitly plotting a graph.

Referring to the plotted graph 400 of FIG. 4, there may be anintersection point 403 obtained at which the rate of removal for both Cuand SiO₂ are substantially the same for a particular sample stage angle.The intersection point may provide the predetermined rate of removal forboth Cu and SiO₂. For example, if a layer comprising Cu and SiO₂ is tobe delayered, then the ion mill operating characteristics may beadjusted in accordance with the stored values, which may relate to thepoint of intersection, to remove both Cu and SiO₂ at the respectivepredetermined rate that would relate to the point of intersection for aperiod of time.

As another example, the rates determined may be for three materials,which may be Cu, SiO₂ and Si₃N₄. The set of ion mill operatingcharacteristics that relate to the rates of removal for Cu, SiO₂ andSi₃N₄ for various sample stage angles may be associated with oneanother, respectively, and stored in any storage medium as would beknown to a worker skilled in the art. Using the predetermined rates andthe associated ion mill operating characteristics from the storagemedium, a graph 500 comprising the predetermined rates as a function ofsample stage angles may be plotted for Cu 501, SiO₂ 502, and Si₃N₄ 503.

Referring to FIG. 5, from the plotted graph, an intersection point 504can obtained at which the rate of removal for both Cu and Si₃N₄ aresubstantially the same for a particular sample stage angle. There mayalso be an intersection point 505, between SiO₂ and Si₃N₄, at which therate of removal for both SiO₂ and Si₃N₄ may be substantially the samefor a particular sample stage angle. For example, if a sample comprisinga layer of Cu, SiO₂ and Si₃N₄ is to be delayered, then the ion milloperating characteristics may be adjusted, in accordance with the storedvalues that relate to the point of intersection for Cu and Si₃N₄, toselectively remove both Cu and Si₃N₄ at the respective predeterminedrate, which would relate to the point of intersection for Cu and Si₃N₄,for a period of time. After which, the ion mill operatingcharacteristics may be adjusted, in accordance with the stored valuesthat relate to the point of intersection for SiO₂ and Si₃N₄, toselectively remove both SiO₂ and Si₃N₄ at the respective predeterminedrate, which would relate to the point of intersection for SiO₂ andSi₃N₄, for a period of time, thus, leaving a substantially uniformunderlying surface. The only ion mill operating parameter that mayrequire change between the intersection points 504 and 505 might be thesample stage angle.

According to some embodiments of the invention, there may not be anyintersection points between any of the materials that are to be removed.For instance, similar to the above example, there may be a layer thatcomprises three materials, A, B and C, to be delayered. After obtainingthe predetermined rates and plotting a graph, similar to FIG. 5, it maybe found that there may not be any intersection points. In such aninstance, the ion mill operating characteristics may be set such thattwo of the three materials with lower predetermined rates may beselectively removed first for a period of time and subsequently the ionmill operating characteristics may be adjusted to selectively remove thethird material with a higher predetermined rate for another period oftime. The time period required for the removal of the first twomaterials may be higher than the time period required for the removal ofthe third material.

As an example, referring to the table below:

Predetermined Predetermined Predetermined Rate of Rate of Rate ofremoval - removal - removal - Material A Material B Material C (A/sec)(A/sec) (A/sec) Ion mill 0.3 0.1 0.3 operating Characteristics - Set IIon mill 0.1 0.3 0.1 operating Characteristics - Set II Ion mill 0.4 0.40.4 operating Characteristics - Set IIIFor a given ion mill operating characteristics—set III the predeterminedrate at which all three materials A, B, and C may be removed is 0.4A/sec. The ion mill operating characteristics may be adjusted to reflectthe ion mill operating characteristics—set III for a certain period oftime, wherein materials A, B, and C are all simultaneously removed. Theunderlying surface, after delayering the three materials A. B, and Csimultaneously, would be substantially uniform. Similarly, the ion milloperating characteristics may be set to reflect the ion mill operatingcharacteristics—set I for a period of time, for example t₁, whereinmaterials A and C are removed at 0.3 A/sec and material B is removed at0.1 A/sec. After which, the ion mill operating characteristics may beset to reflect the ion mill operating characteristics—set II for aperiod of time, for example t₂, wherein materials A and C are removed at0.1 A/sec and material B is removed at 0.3 A/sec. At t=t₁+t₂, all threematerials A, B and C would have been removed equally thereby leaving theunderlying surface substantially uniform.

According to some embodiments of the invention, there may be any numberof materials. Selectively removing one or more materials leavessubstantially uniform or even underlying surface. In addition, theselective removal of the one or more materials itself may be uniform oreven. The selective removal of the one or more materials itself may alsobe non-uniform or uneven. Furthermore, regardless of the selectiveremoval of the one or more materials being uniform or non-uniform, theunderlying surface, after the removal of the one or more materials, issubstantially uniform or even.

According to some embodiments of the invention, one or more operatingcharacteristics or parameters may be adjusted to selectively remove aside-wall of a sample, wherein the side-wall comprises one or morematerials. Predetermined rates, of one or more materials, and theirrespective associated ion mill operating characteristics may be reliedupon to adjust the ion mill to selectively remove the one or morematerials at respective predetermined rates from the side-wall of thesample. In addition, one or more materials surrounding a structure, in asample, may be selectively removed at their respective predeterminedrates by adjusting the one or more ion mill operating characteristics orparameters.

According to some embodiments of the invention, selective removal ofmaterial may take place until a certain thickness is removed. Inaspects, sets of ion mill operating characteristics may be executed in asequential manner to remove one or more materials. The time of executionfor each set of ion mill operating characteristics may be random or maybe set to be a certain value as may be determined previously throughtrial and error via experiments or simulation methods. In addition, thetime of execution for each set of ion mill operating characteristics maybe adjusted in real time by a control system. The control system may usea feedback mechanism to determine the optimal time of execution for eachset of ion mill operating characteristics. The time of execution foreach set of ion mill operating characteristics may be determined basedon the thickness, of the one or more materials, to be removed.

Control System:

According to some embodiments of the invention, a system may comprise anion mill in operative communication with a control system or one or morecontrollers. The control system may control the ion mill componentsindividually or as a whole. The control system may comprise one or moreprocessors with the appropriate software loaded into them to executecontrol. A worker skilled in the art would readily understand thenecessary processors and associated software. The system comprising theion mill in operative communication with the control system may alsocomprise mechanical, electronic and/or optical components for itsworking. These may also be referred to as actuating elements. A workerskilled in the art would readily understand such mechanical, electronicand/or optical components. Furthermore, the control system may beprogrammed to control the system as a whole upon receiving input from auser.

According to some embodiments of the invention, the control system maycomprise, a central control panel or board, at least one computer,processor/microprocessor, or central processing unit (CPU), along withassociated computer software, one or more storage units/devices, powersupplies, power converters, controllers, controller boards, variousprinted circuit boards (PCBs), for example, including input/output (I/O)and D/A (digital to analog) and A/D (analog to digital) functionalities,cables, wires, connectors, shielding, grounding, various electronicinterfaces, and network connectors. The control system may beoperatively connected and integrated with the ion mill and its variouscomponents.

The control system may further comprise one or more storage units tostore predetermined information regarding removal rates for one or morematerials and one or more ion mill operating characteristics orparameters. In addition, the stored information may be predeterminedrates associated with their corresponding sets of ion mill operatingparameters/characteristics for one or more materials. The predeterminedrates may be obtained as explained in various aspects of the disclosureherein.

According to some embodiments of the invention, the control system maybe used to operate the ion mill to perform delayering of a sample byentering appropriate inputs. The inputs may be provided to the controlsystem via a control panel or any input device as would be readily knownto a worker skilled in the art. The control system, upon receivinginputs, adjusts one or more ion mill operating characteristics toperform removal of one or more materials from a sample at theirrespective predetermined rates.

According to some embodiments of the invention, the input may include,but is not limited to the following: an individual value associated withjust one ion mill operating parameter, one or more values associatedwith one or more ion mill operating parameters, the rate of removal ofone or more materials, the delayering thickness or thickness of removalof one or more materials, the time period for removal of one or morematerials, the one or more materials to be removed, sets of ion milloperating characteristics, execution time periods associated with one ormore sets of ion mill operating characteristics, the sequence in whichvarious sets of ion mill operating characteristics are to be executed,or any combinations thereof.

The control system may be programmed to automatically control one ormore ion mill operating characteristics upon receiving an input from auser. For example, the input provided by a user may be to remove Cu froma sample for a certain period of time. Upon receiving this input fromthe user, the control system may, automatically, using the respectivepredetermined information associated with removal of Cu from a storagemedium, adjust the one or more ion mill operating parameters to removeCu for that period of time.

As another example, the user input may be to delayer a certain thicknessof a sample. Upon receiving this input, the control system mayautomatically, using the relevant components of the delayering system,perform the following functions: detect the one or materials that may bepresent on the surface of the layer to be delayered, obtain therespective predetermined information associated with the removal of oneor more materials from that layer, and adjust the one or more ion milloperating characteristics to delayer the sample to the desiredthickness. One or more of the functions may be repeated until thespecified thickness of the sample is delayered.

According to some embodiments of the invention, the control system maycomprise a feedback system. The feedback system may be part of theentire system used for delayering a sample. The feedback system may useany detection mechanism to analyze, in real-time, the delayering of asample. The detection mechanism may detect various facets of thedelayering process as would be necessary to control the operation foroptimal performance. A worker skilled in the art would readily know thevarious facets that need to be detected for optimization of theoperation. The detected facets may be analyzed by the one or moreprocessors. The analyzed results may then be used to automaticallycontrol one or more ion mill operating parameters or sets of operatingparameters to optimally selectively delayer the sample. The detectionmechanism may use a SIMS system. A worker skilled in the art wouldreadily understand various detection mechanisms and detectionsystems/units.

Various Aspects of the Subject Matter Disclosed Herein:

In the instant application, there is further provided a method ofde-layering ICs using an ion beam system, the ICs (or other sample)comprising of one or more materials, wherein rates of de-layering any ofthe one or more materials can be selectively controlled by adjusting oneor more ion beam system parameters, the method comprising the steps ofintroducing an IC (or other sample) in a target path of an ion beam inthe ion beam system, configuring the ion beam system to set each of theone or more parameters at respective predetermined levels that areassociated with predetermined de-layering rates for each of therespective materials in the IC (or other sample); causing the ion beamgun to direct an ion beam at the IC (or other sample).

Other steps may include one or more of the following:

-   -   removing the IC (or other sample) after removal of a layer,        analyzing the IC, repeating;    -   feedback control using a de-layered material detection element        to assess progress of removal of layer or of detected materials        in the sample to automatically and in real-time control        selectivity or non-selectivity of material in any given layer or        sample; and    -   removing substantially uniform layers comprising of homogeneous        or non-homogeneous materials.

In some embodiments, there is also disclosed a method of determiningrelative de-layering rates of various materials using the system of theinstant invention. The method comprises the steps of using the systemdescribed above to de-layer an known material and determine de-layeringrates as a function of one or more parameters (including any interactioneffects in respect of each of the one or more parameters and howinteraction between each such parameter may impact said de-layeringrates); repeating for one or more additional materials; optionallystoring all such empirically gathered information in a data storageelement for use by control system to automatically control selectivitylevels of materials present in any given sample.

In another embodiment, there is provided an ion beam system forselectively de-layering ICs, the ICs comprising of one or morematerials, the ion beam system comprising a chamber, one or more ionbeam sources, and one or more a target stage. The system may optionallycomprise additional elements to introduce various gases into thechamber, target stage control elements, and the following elementsassociated with any of the one or more ion beam sources: apertures,electrostatic lenses (condensors and objective); and all actuatorsnecessary to adjust the one or more parameters.

Other components according to some embodiments of the invention mayinclude one of more of the following:

-   -   Data storage component: for storing predetermined information        regarding parameters and parameter effects on de-layering rates        of materials;    -   Parameter actuator: various actuators on the ion beam system to        automatically, in response to signals received from the control        element, to adjust parameters;    -   Control element (e.g. a computer processor): the control        element, based on predetermined information contained in the        data storage component, and/or information received from the        detection element, configured to send signals to the various        parameter actuators to control adjustment of said parameters;        and    -   De-layering material detection element (e.g. SIMS): this refers        to a detection element capable of detecting or measuring changes        in the sample that relate to de-layering or etching, such as the        existence and quantity of different materials, the existence or        physical, chemical or electrical characteristics of features on        the sample as they become de-layered or material adjacent        thereto becomes de-layered.

Some further aspects of the invention relate to the use of a broad beamion milling system for de-layering layers of semiconductor die on alayer-by-layer basis, including through the methods and systemsdisclosed herein. Such aspects may include the use of an ion millingsystem to remove very thin layers of an IC to expose the underlyingcircuitry across the whole die. Each layer may be made up of a mixtureof materials (metals and dielectrics) in varying shapes and structures.

The ion mill, according to some embodiments of the invention may consistof, for example, a large diameter gridded ion beam source and a variableangle cooled substrate stage that can be tilted and rotated, housed in avacuum pumped process chamber. Various gas injection systems can deliverdifferent process gasses, while a plasma bridge neutralizer is used toneutralize the ion beam. Vacuum gauges, a load-lock, vacuum pumps andcomputer complete the package. SIMS endpoint detection can be installedto help with accurately controlling the etch times.

Other de-layering material detection elements can be used and mayinclude visual detection, other mass spectroscopy tools, or otherchemical detection tools known to a worker skilled in the art.

The parameters of the use of the ion gun and associated target can beadjusted in order to control milling characteristics. Some of theseparameters, which may affect the beam conditions, chamber conditions,sample conditions and orientation, and use of other materials in thebeam or other material streams directed to the sample, can affectdifferent milling characteristics. For example, by controlling the beamconditions (accelerating voltage, current), the gasses used (amount,type, and precisely where they are injected), the gun-to-sample angle,sample cooling and sample rotation speed, the milling characteristics ofthe semiconductor die materials may be controlled. Other parameters mayinclude one or more of the following:

-   -   Chamber—base pressure, cross-over pressure, process pressure;    -   Load-lock base pressure, cross-over pressure;    -   Stage—linear location, angle, rotation speed, temperature;    -   Ion Source—RF power (fwd/reflected), accelerator voltage,        accelerator current, beam voltage, beam current, extractor grid        configuration (material, size, number and grid pattern of the        extractor grids);    -   Plasma bridge neutralizer—cathode voltage, emission current;    -   Gas injection—PBN gas flow, source gas flows and types (could be        multiple, could be inert or reactive), background gas flows and        types (could be multiple);    -   Time—process step(s) time(s), number and order of steps (could        use a series of different process conditions in a particular        sequence);    -   Angle of the sample-to-gun (covers both moveable sample and        moveable gun) is adjusted to select optimum etching rates and        sidewall angle; and    -   Any of the parameters in known ion beam systems.

Selectivity (differential etch rate) between different materials used inthe circuit layouts or other samples may depend on all, or some of thoselisted parameters and/or interaction effects therebetween. They can beadjusted and manipulated to allow selective etching (removing only aspecific material type) or non-selective etching (removing all materialat similar rates), allowing for accurate and precise removal of thehomogenous and non-homogenous layers of an integrated circuit. Thethickness of the removed layer, whether homogeneous or non-homogeneous,can therefore be made uniformly or substantially uniformly (and in anycase significantly less than any stratum deposited in an IC).

The rate of removal for any given material as a function of one or moreparameters can be predetermined and stored in a data storage componentthat is associated with systems and devices for carrying out the subjectmatter disclosed herein. This data may be collected based on empiricalmeasurements that are systematically carried out by adjusting one ormore parameters and then measuring de-layering rates for a particularmaterial. By carrying out multi-factored analyses that assessde-layering rates relative to changes in various parameters, theinteractive effects between such changes can also be predetermined andstored in the data storage component. Accordingly, the ion beam systemcan be configured to use the predetermined parameter de-layeringparameter effects to carry out de-layering with a precise level ofdesired selectivity.

In some embodiments, the methods and systems disclosed herein are usedto de-layer a target in a way that leverages the ability to selectivelyremove or etch layers that comprise multiple materials and arenon-homogeneous. One application of this use is to provide for greaterease of visual and other types of analysis layer by layer. This isparticularly useful in reverse engineering applications. While it may bean object to remove a layer of very uniform thickness across anon-homogenous target in order to analyze such layers underneath, it mayalso be an object to remove material surrounding a structure of adifferent material in order to more easily analyze (visually,electrically or otherwise) a particular material within a given layer.In other examples, the systems and methods disclosed herein may be usedfor deposition of materials onto the target to protect or pre-processthat layer so that analysis or reverse engineering for a given layer ismade possible. For example, once sufficient layers have been removed foranalysis of a particular layer, it may be necessary to protect a layerfrom ambient conditions outside the ion beam chamber, and to expose thetarget for subsequent analysis of a particular layer. In other cases,material in a given layer may be pre-processed in order to make it moreamenable to (a) etching at rates similar to other materials in the givenlayer that is non-homogeneous including etching using ion beam mill orother types of etching known in the art; and/or (b) analysis, visual orotherwise (for example, the surface characteristics of one material in aparticular structure may be roughened or have material adsorbed orabsorbed thereto to facilitate analysis). All of these objectives can beselectively managed and controlled by controlling the various parametersaccording to the systems and methods disclosed herein.

According to another embodiment, the parameters are controlled to managethe relative side-wall etching rates versus the de-layering etchingrates. This may include, in some aspects, minimizing sidewall etching ina direction perpendicular to the direction of delayering.

The various parameters can also be used to control reaction ratesselectively with various materials present on a sample when reactivespecies are introduced into the chamber and/or used as or mixed with thematter ejected from the ion beam source. Other objectives that can beachieved by controlling the parameters include the changes to surfaceroughness, chemical damage/modification, and physicaldamage/modification.

Some embodiments of the systems and methods described herein areconfigured to etch, deposit, or pre-process material from a sampleuniformly across samples, even when the sample is large or very large,relative to the depth of the beam effect.

The parameters can be adjusted to selectively remove certain materialsat different rate relative to other material that may be found in agiven layer, so as to remove some materials and not others.Alternatively, the parameters can be adjusted to ensure substantiallyequal rates of de-layering to evenly remove extremely thin layers acrosslarge areas (in particular where the thickness of the layer removed ismuch smaller than the length or width of the sample in the plane of thelayer). In some applications, the length and/or width may be in therange of 5-20 centimeters and the thickness of the removed layer may bein the range of picometers to nanometers.

Control system using feedback control of the SIMS measurement todetermine rate of delayering and presence of de-layered material, orchanges thereof. This can be used as part of a feedback control systemto cause de-layering of desired materials at desired rates.

EXEMPLARY EMBODIMENTS Vacuum System:

Appropriately sized vacuum chamber to support automatic sampleload/unload through an isolation lock. Accommodating the sample stage,ion-source, pumping ports, gas supply ports and ancillary viewing andmonitoring features.

Fast pumping capability supporting process pressures in the 5e−4 regimewith a leak rate less than 5e−5 torr l/s.

Sample Stage:

Capable of accepting a 4″ diameter sample. Temperature controlled(cooled) to allow constant temperature operation. Thermal conductionfrom sample to stage. Rotation of sample about the central axis, andtilting in the plane of the ion-source from normal (+/−90 deg) to 0degrees.

Shutter control protection of the sample during run-up andprocess-setting changes.

Ion Source:

12 cm diameter RF ICP auto impedance-matched with dual-grid focusedoutput. Control of accelerating voltage, extraction voltage and beamcurrent in the range of about 0-1000V, 0-500 mA.

Gas Supply:

MFC controlled ion-source supply of noble or reactive gas and chamberbackground gas for chemical-mill enhancement. Also supply for PBN.

PBN:

Argon supplied ionizer injecting an electron stream for ion-beamneutralization.

De-Layering:

Every surface factor affects the milling process—material, topography,roughness, feature width, spacing, and the influence is dynamic innature. The instant ion beam milling system is configured to matchmaterial rates as much as possible, and deal with feature topographythrough multiple beam angle/current application steps. One way tode-convolve all of these different effects from adjustments toparameters is to monitor effects by “trial and error” as much aspredictable rate matching.

Another difficulty is to know where in the vertical structure theprocess is at any given time. A “material monitor” can be used todetermine variations in the material mix and capturing new materials asthey are uncovered—i.e. end-point detection. A SIMS detector can beconfigured to be a “material monitor”.

Another tool for influencing etch rates/selectivity is the use ofadditional or substitutional gasses either directly in the ion-source(reactive), or as a background gas fragmented by the ion-beam then ableto selectively react on the surface.

Each step of the method disclosed herein may be executed on anycomputing device, such as a personal computer, server, PDA, or the likeand pursuant to one or more, or a part of one or more, program elements,modules or objects generated from any programming language, such as C++,Java, C, or the like. In addition, each step, or a file or object or thelike implementing each said step, may be executed by special purposehardware or a circuit designed for that purpose.

1-22. (canceled)
 23. A non-transitory computer storage medium storingcomputer-executable instructions that, when executed by a processor,cause the processor to perform the following method: identifying aplurality of materials in an exposed surface of a sample; defining aremoval sequence defined by distinct sets of one or more predeterminedoperational characteristics, wherein each of said sets corresponds withrespective ion beam removal rates for each of said materials, each ofsaid removal rates being greater than zero; operating an ion beam millin accordance with the defined sequence to simultaneously remove, duringeach distinct set of one or more predetermined operationalcharacteristics, using an ion beam from the ion beam mill, each of theplurality of materials at said respective ion beam removal rates suchthat upon completion of said defined sequence, a substantially planarlayer of substantially constant thickness has been removed from theexposed surface of the sample; acquiring surface data from a newlyexposed surface of the sample; and repeating the method for at least onemore layer, the acquired surface data for reverse engineering at least aportion of the sample; wherein said one or more predeterminedoperational characteristics comprise at least one of: angle ofsample-to-ion beam direction, ion beam size, ion type, sample stagetemperature, chamber base pressure, chamber cross-over pressure, chamberprocess pressure, sample stage linear location, sample stage angle,sample stage rotation speed, ion source accelerator voltage, ion sourceaccelerator current, ion source beam voltage, ion source beam current,ion source extractor grid configuration, ion source extractor gridmaterial, ion source RF power, extraction voltage, plasma bridgeneutralizer (PBN) cathode voltage, PBN emission current, PBN gas flow,ion source type, ion source gas flow rate, chamber background gas type,chamber background gas flow rate, or sidewall angle.
 24. Thenon-transitory computer storage medium of claim 23, wherein said removalsequence comprises defining distinct removal times for at least two ofsaid distinct sets.
 25. The non-transitory computer storage medium ofclaim 23, further comprising producing hierarchical circuit schematicsusing the acquired surface data.
 26. The non-transitory computer storagemedium of claim 23, wherein the ion beam operates in the presence of areactive gas.
 27. The non-transitory computer storage medium of claim23, wherein the ion beam operates in the presence of a non-reactive gas.28. The non-transitory computer storage medium of claim 23, wherein saidion beam mill is one of a broad ion beam mill and a focused ion beammill.
 29. The non-transitory computer storage medium of claim 23,wherein a thickness of said planar layer is smaller than a length orwidth of said planar layer.
 30. The non-transitory computer storagemedium of claim 23, wherein a surface area of said planar layer is inthe range of 5 to 20 square centimeters.
 31. The non-transitory computerstorage medium of claim 30, wherein the ion beam operates in thepresence of a reactive gas, and wherein the predetermined operationalcharacteristics further comprise at least one of a type of reactive gas,and a flow rate of reactive gas.